JPS63260371A - Audio multiplex demodulation circuit - Google Patents

Abstract

PURPOSE:To eliminate noise due to buzz without incurring deterioration of stereo separation by providing a means eliminating a noise of 60Hz in a stereo difference signal (L-R) between a subchannel demodulation circuit and a matrix circuit in response to white level. CONSTITUTION:A filter 6 inserted between a subchannel demodulation circuit 3 and a matrix circuit 5 and a white level detection circuit 7 detecting the white level of the detected video signal are provided. Thus, the noise due to buzz in the demodulated subchannel signal is eliminated by the filter 6 and the filter characteristic in the filter 6 is controlled in response to the white level in the video signal, thus the noise is much effectively eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to an audio multiplex demodulation circuit, and more particularly to an audio multiplex demodulation circuit that reduces noise in subchannel signals caused by buzz.

(b) Prior art The TV (television) audio multiplex signal currently broadcast in the United States is made by frequency multiplexing a main channel signal, a stereo pilot signal, a subchannel signal, a second audio signal, etc. For example, in the case of stereo broadcasting, the main channel signal is composed of a stereo sum signal (L+R), and the subchannel signal is composed of subcarrier frequencies (31, 46
8KHz, equivalent to 2f'R) is converted into a stereo difference signal (L-R
) is AMDSB modulated. The US TV audio multiplex signal, which has been FM modulated on the audio carrier wave of the TV broadcast signal, can be extracted from the IF detection circuit of the TV receiver. The frequency of the stereo pilot signal in the US TV audio multiplex signal is f'H, and the ft
t stereo pilot signals are applied to the PLL circuit,
A subcarrier of frequency 2f'tt is regenerated. Then, the stereo difference signal (L-R) is demodulated using the subcarrier, and the stereo difference signal (L-R) and the stereo sum signal (L+R) are matrixed to form left and right stereo signals, L and R is demodulated.

Incidentally, the demodulation circuit for the US TV audio multiplex signal is, for example, 'NE
C Technical Journal JVO1, 39NQ3/1986, pages 4 to 7.

(c) Problems to be solved by the invention By the way, a TV receiver extracts a 4.5 MHz audio beat signal and performs FM detection to obtain the audio signal, but how to extract the audio beat signal? There are two methods: a method in which a video detection circuit is also used to extract the signal (intercarrier method), and a method in which the video signal and audio signal are separated and then detected (split carrier method). In the split carrier method, the video and audio are separated and then detected, so there is little interference from the video signal to the audio signal, but in the intercarrier method, the audio beat signal is detected from the video detection circuit, so the video and audio signals are detected. The signal is mixed into the audio signal, and the audio beat signal is abnormally modulated by the video signal. In particular, in the American audio multiplex broadcasting system, the stereo pilot signal has a frequency of r14 and the subcarrier frequency has been selected to be 2fH. )
12) and the subcarrier, causing buzz on the same channel. In an audio multiplex demodulation circuit that uses a PLL circuit to reproduce subcarriers and demodulate a stereo difference signal (LR), when the stereo pilot signal is affected by the buzz, a phase shift occurs in the PLL circuit. The phase-shifted 2fH subcarriers are applied to the subchannel demodulation circuit. Then, a stereo difference signal (LR) whose demodulation level fluctuates in accordance with the phase shift is generated as an output of the subchannel demodulation circuit. Said P
The horizontal synchronization signal applied to the LL circuit has a vertical synchronization signal period every 60 Hz, and since there is no horizontal synchronization signal during the vertical synchronization signal period, the amount of phase shift changes. Then, during the period, the subcarrier 2r, which has a different phase from the subcarrier 2f, is applied to the subchannel demodulation circuit, and a stereo difference signal (L-R) is generated at a demodulation level different from the demodulation level. . The situation is shown in Figures 2 (a) and (b). Figure 2 (A) shows the horizontal synchronizing signal applied to the PLL circuit, and Figure 2 (B) shows the demodulated stereo difference signal (
LR). The horizontal synchronizing signal in FIG. 2(a) does not exist during the vertical period.

Therefore, the amount of phase shift of the PLL circuit changes between the horizontal period and the vertical period, and the demodulation level changes. Period A in FIG. 2(b) indicates the demodulated output in the vertical period, and period B indicates the demodulated output in the horizontal period. Here, the demodulated stereo difference signal (
LR) is applied to the matrix circuit, but as is clear from the figure, the stereo difference signal (LR) contains 60H.
Since z noise is mixed in, 60 Hz noise is also generated during the demodulation output of the matrix circuit.

Furthermore, since the second harmonic of the horizontal synchronization signal that is mixed separately from this is mixed into the subcarrier, noise is also generated in the demodulated stereo difference signal (LR) itself. In this case as well, as a result, 60H2 noise appears. Such 60 Hz noise occurs especially when the amplitude of the horizontal synchronizing signal is large, that is, when the white level in the video signal is high.

In this way, 60Hz noise caused by buzz is generated in the stereo difference signal (L-R), and 60Hz noise and its harmonics are generated in the left and right stereo signals after matrixing. There was a problem.

(d) Means for Solving Problems The present invention has been made in view of the above points, and includes a subchannel demodulation circuit that demodulates a subchannel signal in a detected audio multiplex signal, and a subchannel demodulation circuit for demodulating a subchannel signal in a detected audio multiplex signal. In the audio multiplexing demodulation circuit, the audio multiplexing demodulation circuit includes a matrix circuit for matrixing a main channel signal in the subchannel demodulation circuit and a subchannel signal obtained from the subchannel demodulation circuit, a filter inserted between the subchannel demodulation circuit and the matrix circuit; The present invention is characterized in that it includes a white level detection circuit that detects the white level of the detected video signal.

(e) Effects According to the present invention, the noise caused by buzz in the demodulated subchannel signal is removed by the filter, and the filter characteristics of the filter are controlled according to the white level in the video signal. The noise can be removed more effectively.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which (1) is a tuner that receives an RF signal and converts it to an IF signal;
) is an IF detection circuit that amplifies and detects the IF multiplied signal from the tuner (1), (3) is a subchannel demodulation circuit to which the audio multiplexed signal from the IF detection circuit (2) is applied, and (4)
(5) is a PLL circuit to which a stereo pilot signal of frequency rH in the audio multiplexed signal is applied as a reference signal and reproduces a subcarrier of frequency 2f'H; (5) is a stereo difference signal (L+R) and a subchannel demodulation circuit ( 3) is a matrix circuit that matrixes the stereo difference signal (L-R) and demodulates the left and right stereo signals, and (6) is a sub-channel demodulation circuit (6) that generates the stereo difference signal (L-R).
A filter for removing a 60Hz component is provided between the output terminal of 3) and the matrix circuit (5), and a filter for removing a 60Hz component is applied to the video signal from the IF detection circuit (2), and the filter for removing the 60Hz component is This is a white level detection circuit that detects white level.

Next, the operation will be explained. RF received by antenna (8)
The doubled signal is applied to the IF detection circuit (2) via the tuner (1) and detected. A video signal is output to the terminal (9), and an audio signal is output to the terminal (10). Then, the audio signal, that is, the stereo sum signal (L
+R), a stereo difference signal (LR), a stereo pilot signal, etc., is applied to a subchannel demodulation circuit (3).

Further, the video signal from the terminal (9) is applied to a white level detection circuit (7), and its white level (luminance signal level) is detected. The white level detection circuit (
7) is as shown in FIG. 3, for example.

In Fig. 3, a video signal is applied to the input terminal (20), and after the DC component is removed via the DC blocking capacitor (21), only the positive signal is detected by the diode (22) and the capacitor (23) is charged to. Here, when the white level is close to the reference white level, the amplitude of the horizontal synchronizing signal becomes large, so the terminal voltage of the capacitor (23) increases. Also, when the white level is far from the reference white level, the horizontal synchronizing signal Since the amplitude of is small,
The terminal voltage of the capacitor (23) decreases. Therefore, a voltage corresponding to the white level is generated at the output terminal (24). On the other hand, the stereo pilot signal of frequency fM in the audio multiplex signal is applied to the PLL circuit (4), and the subcarrier of 2f'H is reproduced. Therefore, depending on the subcarrier from the PLL circuit (4), the subchannel demodulation circuit (3) uses the frequency 21. The stereo difference signal (L-R) modulated is demodulated. As a result, the stereo difference signal (L
-R) occurs.

The stereo difference signal (LR) contains 60 Hz noise due to the factors mentioned above. However, in the circuit shown in Fig. 1, the subchannel demodulation circuit (3
) and the matrix circuit (5), the filter (6) for removing the 60 Hz component is inserted, so the noise is removed. The level of the noise changes depending on the white level of the video signal. Therefore, the present invention detects the white level and controls the filter characteristics of the filter (6) based on the detection result to more effectively reduce the noise. The situation will be explained with reference to FIG.

First, when the white level is high, an output signal of r H level is generated from the white level detection circuit (7) and applied to the control terminal (25) in FIG. Then, in response to the "H" level output signal, the first VCA (26)
(voltage control amplifier) is turned on and the second VCA (27) is turned on. Therefore, the stereo difference signal (L-R) from the input terminal (28) is passed through the bypass filter (29) to 60H.
The noise component of z is removed and generated at the output terminal (30). Furthermore, when the white level is low, an output signal of "L" level is generated from the white level detection circuit (7) and applied to the control terminal (25). Then, the above r
The first VCA (26) is turned on and the second VCA (27) is turned off in response to the output signal of L level. Therefore,
The stereo difference signal (L-R) is transmitted to the first VCA (26).
, it is output to the output terminal (30) as it is (7).Furthermore, if the white level is approximately between the "H" level and the "L" level, an output corresponding to the "H" level and the "L" level is output. A signal is applied to the control terminal (25) of FIG. 4. Then, depending on the level of the output signal, the first and second V
The gains of CA (26) and CA (27) are switched, and the mixing ratio of both signals in the adder circuit (31) is controlled.

Therefore, the noise removal rate of the output signal that can be led out to the output terminal (30) corresponds to the level of the output signal. In the embodiment shown in FIG. 4, the filter characteristics are substantially adjusted by adjusting the level of the output signal after passing through the filter, but the bypass filter (29
) The time constant of the white level detection circuit 7) may be changed according to the output signal of the white level detection circuit 7).

By the way, if the filter (6) is inserted into the signal path of the stereo difference signal (L-R), the stereo difference signal (L-R)
Some of the signals in the middle are also reduced, and stereo separation in the low range becomes slightly worse. Therefore, the present invention detects the white level and removes noise using a filter only when noise appears significantly.

Therefore, according to the circuit of FIG. 1, the output terminal (12) and (
13) Left and right stereo signals with significantly reduced noise can be obtained.

(G) Effects of the Invention As described above, according to the present invention, a stereo difference signal (L-R) is transmitted between the subchannel demodulation circuit and the matrix circuit.
Since a means for removing the 60 Hz noise in the image according to the white level is provided, the noise caused by buzz can be removed without deteriorating the stereo separation.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a characteristic diagram for explaining the generation of noise caused by buzz, and FIG.
FIG. 4 is a circuit diagram showing a specific example of a white level detection circuit, and FIG. 4 is a circuit diagram showing a specific example of a filter. (2)...IF detection circuit, (3)...Sub, channel demodulation circuit, (5)...Matrix circuit, (
6)...filter, (7)...white level detection circuit.

Claims (1)

[Claims] (1) A subchannel demodulation circuit that demodulates a subchannel signal in the detected audio multiplexed signal, and a matrix circuit that matrixes the main channel signal in the audio multiplexed signal and the subchannel signal obtained from the subchannel demodulation circuit. In an audio multiplex demodulation circuit comprising:
A filter inserted between the subchannel demodulation circuit and the matrix circuit and a white level detection circuit for detecting the white level of the detected video signal are provided, and the white level detection circuit detects the white level of the detected video signal according to the output signal of the white level detection circuit. An audio multiplexing demodulation circuit characterized in that noise caused by buzz in the subchannel signal is removed by controlling filter characteristics of a filter.